This invention is directed toward the preparation of stabilized, rare earth metal-doped cadmium halide glasses exhibiting excellent transmission far into the infrared region of the electromagnetic radiation spectrum.
Glasses transmitting in the infrared portion of the radiation spectrum have long been of interest as potential candidates for ultra-low loss optical fibers. More recently it has been recognized that these same materials, when doped with rare earth metals, can demonstrate properties rendering them eminently suitable for making efficient lasers, amplifiers, and upconverters.
The peculiar characteristic of a material that imparts the ability to transmit radiation far into the infrared region is the same one which makes the material a good host of rare earth metals for the three above-mentioned applications. That singular property comprises a low fundamental vibrational frequency (phonon energy) which is typically associated with the strongest chemical bond of the material matrix. For example, the more ionic metal fluorine bond in a fluoride material is weaker than the more covalent metal oxygen bond in an oxide material which, in turn, results in the fluoride material exhibiting superior infrared transmission Moreover, in a given class of materials, bonds are weaker and, hence, infrared transmission is improved, when the component ions are heavy and/or have low valence. Thus, within the particular class of halide glasses, the cadmium-based glasses have more extended transmission of infrared radiation than the well known glasses based on either BeF.sub.2 or ZrF.sub.4.
Radiative emission of rare earth metal-doped materials is based upon electronic transitions between atomic-like 4f levels that are localized on the rare earth metal ions. From a particular level radiative emissions compete with non-radiative emissions. Often the dominant non-radiative process is the emission of one or more phonons. The probability of this non-radiative process is known to decrease exponentially with the number of phonons required to bridge the energy gap of the transition. Therefore, materials with low-phonon energies will require more phonons to bridge that gap and, hence, will have less competitive non-radiative emission processes and higher radiative efficiencies, thereby enhancing any process which depends upon fluorescence between rare earth metal levels.
Three basic devices can be made with rare earth metal-doped materials, viz., lasers, amplifiers, and upconverters. Lasers demand good radiative efficiency for the laser transition. Amplifier efficiency is similarly directly tied to radiative efficiency. In fact, only those transitions separated by more than twice the phonon energy of the host material can be made to lase or amplify. Consequently, the lower the material phonon energy, the larger the number of transitions that can be made to lase or amplify. In an upconversion device light is converted from a longer wavelength, e.g., in the infrared, to a short wavelength, e.g., in the visible. Such a process can be used both for detecting long wavelength light and as a source of short wavelength light. The simplest upconversion process is generally a two-step process, first involving the absorption of a long wavelength photon followed by the promotion of an electron to an intermediate level. From this intermediate level the electron is further promoted to a higher level through the absorption of a second photon (so-called excited state absorption) or through the exchange of energy with another excited rare earth metal ion (so-called energy transfer upconversion). Clearly, the efficiency of emission from the final state will be positively correlated with the radiative efficiency of the transition from the upper state. In addition, the efficiency will also be positively correlated with the lifetime of the electrons in the intermediate state. The longer the lifetime (before the electron decays back towards the ground state), the longer a given electron will be available for excited state absorption energy transfer upconversion and, therefore, the greater the probability of such a promotion. Accordingly, as the probability of the non-radiative processes is reduced in a material with a lower phonon energy, the excited state lifetimes are longer, thereby rendering the multi-step upconversion process more efficient.
Cadmium halide-containing glasses comprise one class of glasses with very low phonon energies. The cations present in those glasses are monovalent and/or divalent. Moreover, the most stable cadmium halide glasses contain a mixture of light (fluoride) and heavier (chloride) anions. As a result, these glasses have more extended transmissions in the infrared than do other halide glasses based upon zirconium, hafnium, uranium, indium, and/or aluminum fluorides.
Nevertheless, unlike heavy metal fluoride glasses, the cadmium halide glasses become less stable with the addition of trivalent cations, such as the rare earth metals. For example, the addition of as little as 0.1 mole percent of praseodymium halide is sufficient to convert an otherwise stable and clear cadmium chloride-containing glass into an opaque, partially devitrified material. This behavior was observed by Jha and Parker in "Preparation of Infrared Transmitting CdF.sub.2 Based Mixed Halide Glasses", Physics and Chemistry of Glasses, 32, No. 1, February 1991, pages 1-12. Thus, at page 7 the author stated:
Our results indicate that trivalent fluorides cannot be incorporated into mixed halide glasses as they can into heavy metal fluoride glasses. LaF.sub.3 and YF.sub.3 caused crystallization while AlF.sub.3 dissolves to a limited extent, but at the expense of the infrared cutoff edge (see next section) and even in this case only 2 mol % could be added without significant crystallization. Similarly, BiCl.sub.3 cannot be incorporated in these glasses, which seems to suggest that it is either the chemical nature or the size of these trivalent halides that decides their solubility.
The primary objective of the present invention was to discover means for producing stable, transparent cadmium halide glasses containing trivalent rare earth metal cations, which glasses exhibit excellent transmission far into the infrared portion of the radiation spectrum, thereby commending their utility in the fabrication of efficient lasers, amplifiers, and upconverters.